1. Field of the Invention
This invention relates to an organic electro-luminescence display (ELD), and more particularly to a driving apparatus for an organic electro-luminescence display device that is adaptive for reducing a deterioration of organic light-emitting diode device in the organic electro-luminescence display device.
2. Description of the Related Art
Recently, there have been developed various flat panel display devices reduced in weight and bulk that is capable of eliminating disadvantages of a cathode ray tube (CRT). Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an electro-luminescence (EL) display, etc. device.
In such flat panel display devices, the PDP has the most advantage for making a large dimension screen because its structure and manufacturing process are simple, but has a drawback in that it has low light-emission efficiency and large power consumption. The LCD has a difficulty in making a large dimension screen because it is fabricated by a semiconductor process, but has an expanded demand as it is mainly used for a display device of a notebook personal computer. However, the LCD has a drawback in that it has a difficulty in making a large dimension screen and it has large power consumption due to a backlight unit. Also, the LCD has characteristics of a large light loss and a narrow viewing angle due to optical devices such as a polarizing filter, a prism sheet, a diffuser and the like.
On the other hand, the EL display device is largely classified into an inorganic EL device and an organic EL device depending upon a material of a light-emitting layer, and is a self-luminous device. When compared with the above-mentioned display devices, the EL display device has advantages of a fast response speed, large light-emission efficiency, a large brightness and a large viewing angle. The organic EL display device can display a picture at approximately 10[V] and a high brightness of ten thousands of [cd/m2].
FIG. 1 is a schematic section view showing a structure of a conventional organic EL display device.
In the organic EL display device 1, as shown in FIG. 1, an anode electrode 2 is formed from a transparent electrode pattern on a substrate 1. On the substrate 1, a hole carrier layer 3, a light-emitting layer 4 formed from an organic material, an electron carrier layer 5 and a cathode 6 made from a metal are disposed.
FIG. 2 is a circuit diagram of a driving apparatus for the conventional organic EL display device, and FIG. 3 is a circuit diagram for explaining an operation principle of an organic light-emitting diode device in the organic EL display device shown in FIG. 2. Further, FIG. 4 is a driving waveform diagram of the organic EL display device shown in FIG. 2.
Referring to FIG. 2 to FIG. 4, the driving apparatus for the conventional organic EL display device includes a data voltage source Vdata connected to an anode of an organic light-emitting diode device 20, first and second scan voltage sources Vin1 and Vin2 connected to a cathode of the organic light-emitting diode device 20.
The data voltage source Vdata supplies a positive voltage to data lines DL1 to DLm of the organic EL display device while the first and second scan voltage sources Vin1 and Vin2 supply a negative voltage and a positive voltage to scan lines SL1 to SLn of the organic EL display device.
Generally, the driving apparatus for the organic EL display device applies the same voltage to the data voltage source Vdata supplying a positive voltage to the data lines DL1 to DLm and the second scan voltage source Vin2 supplying a positive voltage to the scan lines SL1 to SLn. A ground voltage GND is applied to the first scan voltage source Vin1 supplying a negative voltage to the san lines SL1 to SLn.
Further, the driving apparatus includes switching devices 21 connected between the anode of the organic light-emitting diode device 20 and the data voltage source Vdata, and first and second switching devices 22 and 23 connected between the cathode of the organic light-emitting diode device 20 and the first and second scan voltage sources Vin1 and Vin2, respectively.
The first switching devices 22 are sequentially turned on in response to a control signal T1 to thereby sequentially apply a scanning pulse SCAN having a negative voltage, that is, a forward voltage to the scan lines SL1 to SLn. A data pulse DATA is synchronized with the scanning pulse SCAN applied to the scan lines SL1 to SLn to be applied to the data lines DL1 to DLm as a positive voltage.
More specifically, as the first switching device 22 connected to the first scan line SL1 is turned on in response to the control signal T1, the scanning pulse SCAN is applied to the first scan line SL1 as a negative voltage. At the same time, the data pulse DATA is applied to the data lines DL1 to DLm as a positive voltage. When a negative voltage is applied to the first scan line SL1 and a positive voltage is applied to the data lines DL1 to DLm, the organic light-emitting diode device 20 at the first line is emitted by a forward bias. Thereafter, as the second switching device 23 connected to the first scan line SL1 is turned on in response to a control signal T1, the scanning pulse SCAN is applied to the first scan line SL1 as a positive voltage. While the control signal T2 supplying a positive voltage to the first scan line SL1 and the first control signal T1 supplying a negative voltage to the second scan line SL2 being applied, the organic EL display device sequentially emits a light to display a picture.
FIG. 5 is a detailed view of the A portion shown in FIG. 4.
Referring to FIG. 5, when the scanning pulse SCAN is switched from a negative voltage into a positive voltage, an overshoot phenomenon caused by the switching emerges from the scanning pulse SCAN. Such an overshoot phenomenon causes a deterioration of the organic light-emitting diode devices 20. This appears more seriously as a level of the positive voltage applied to the cathode of the organic light-emitting diode device 20 shown in FIG. 5 goes higher.
If the second scan voltage source Vin2 is supplied with a lower voltage than the data voltage source Vdata so as to reduce the overshoot phenomenon, then a voltage of the data voltage source Vdata supplied to the anode of the organic light-emitting diode device 20 becomes larger than that of the second scan voltage source Vin2 supplied to the cathode of the organic light-emitting diode 20. In this case, there is raised a problem in that, as the organic light-emitting diode devices 20 at the selected lines as well as the remaining organic light-emitting diode devices 20 of the organic EL display device are forwardly biased, a light-emission is made while a current flowing in all the organic light-emitting diode devices 20.